Thanks to Island star-expert Dr Lucy Rogers of the Vectis Astronomical Society for sending this piece in. Now we know how often once every Blue moon is! Sounds like it’s time to get your telescopes out tonight – Ed
There will be a partial eclipse of a blue moon on 31st December. However, only a very small area of the Moon will enter the Earth’s shadow, and although there may be some shading seen across the whole of the Moon, it won’t be the colour blue. In recent American parlance the term blue moon has come to mean two full moons in a calendar month.
Tonight _is_ once in a blue moon
These blue moons occur about every two and a half years. The last time this happened was in June 2007 and the next time will be in August 2012. In 1999 there were two full Moons in January, and again in March, but no full Moon in February.
A blue moon will never happen in a February, as the moon takes about 29 1/2 days to go through its phases. For February to have no full moon at all is more rare than a month to have two full moons. It happens about four times every century. The next time February does not have a full moon will be 2018.
The moon can turn blue due to atmospheric effects but this phenomenon is rare. Moons that do appear blue are caused by high-altitude dust in the earth’s atmosphere. The dust scatters the red wavelengths of the light coming from the sun. The blue colours reach the eye but the red colours are lost. This is similar to why the sun appears orange-red at sunset – the air molecules in the atmosphere scatter the blue light and only the orange-red light is seen. Blue moons were seen in 1883 after the eruption of the volcano Krakatoa and in 1950 because of the forest fires in Canada.
“The two full moons in a month blue moon misnomer appears to have its origins in an article in Sky and Telescope magazine in 1946″ said Ian Morison from Jodrell Bank Observatory “and it was probably a mistake – the author had misinterpreted a page in the Maine Farmers’ Almanac.”
Blue Moon defined
The author of the Sky and Telescope article, James Hugh Pruett, had said “Seven times in 19 years there are 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called blue moon.”
“However, in the almanac the term was actually taken to mean the third of four full moons in a season – spring, summer, autumn or winter. Usually there are only three full moons in a season. These events would often be highlighted with a blue symbol – hence the name blue moon,” added Mr Morison. The date of the last full moon in a season had significance to the ecclesiastical calendar. Festivals, such as Easter, were determined by the positions of the sun and the full moon.
“Even if the calendrical meaning is new, I don’t see any harm in it,” said Sky & Telescope’s founding editor, Charles A. Federer Jr. on their web page. “It’s something fun to talk about, and it helps attract people to astronomy.”
Another definition, and probably the one in most common use, is listed in the Oxford English Dictionary as “a rarely recurring period”. The first recorded use was in 1821. However, the saying has been around for far longer “Perhaps for 400 years,” said Mr Morison. “Originally it referred to an obvious absurdity and thus would never happen.”
Image: different2une under CC BY 2.0
